fixes semi-regression; discard check now skips nkHiddenSubConv (#23840)

follow up https://github.com/nim-lang/Nim/pull/23681

ref https://forum.nim-lang.org/t/11987

(cherry picked from commit 648f82c2ed)

compiler/semstmts.nim

@@ -132,7 +132,7 @@ const
   skipForDiscardable = {nkStmtList, nkStmtListExpr,
     nkOfBranch, nkElse, nkFinally, nkExceptBranch,
     nkElifBranch, nkElifExpr, nkElseExpr, nkBlockStmt, nkBlockExpr,
-    nkHiddenStdConv, nkHiddenDeref}
+    nkHiddenStdConv, nkHiddenSubConv, nkHiddenDeref}
 
 proc implicitlyDiscardable(n: PNode): bool =
   # same traversal as endsInNoReturn

tests/discard/tdiscardable.nim

@@ -140,3 +140,12 @@ block: # issue #14665
         continue
       inc i
   test()
+
+block:
+  proc test(): (int, int) {.discardable.} =
+    discard
+
+  if true:
+    test()
+  else:
+    quit()